The present invention relates to die assemblies for the extrusion of thermoplastic pipe. More specifically, the present invention relates to die assemblies for the extrusion of thermoplastic pipe from plastics having an appreciable amount of memory.
It is common to manufacture structures, from samll tubing with a small diameter up to pipe with diameters as large as 36 inches, from thermoplastics. Thermoplastic pipe has found uses in municipal water and sewer systems, for the transport of natural gas and the like, for the transport of fluids of all types in chemical plants and refineries, etc., depending, of course, upon the resistance of the plastic to deterioration by the fluid transported as well as the environment around the outside of the pipe. For uses such as water and sewer lines, natural gas lines and the like, such pipe must, of course, have certain minimal pressure ratings. Accordingly, the American Society for Testing Materials (ASTM), as well as other standardizing organizations, have set minimal standards for plastic pipe. The pressure rating of a plastic pipe is related to what is termed the "Standard Dimension Ratio" (SDR) of the pipe, which may range from SDR 11 to as high as SDR 32.5 and cover pressure ranges from about 160 psi to 50 psi, respectively. The Standard Dimension Ratio (SDR) is equal to the nominal outside diameter (OD) of the pipe divided by the minimum wall thickness of the pipe. Thus, the following formula defines SDR: EQU SDR=Dp/t (1)
where:
Dp=Nominal OD of Pipe, inches, and PA1 t=Minimum Wall Thickness, inches. PA1 P=Pressure Rating, psi, at 73.4.degree. F. and PA1 S=Hydrostatic Design Stress, 800 psi.
Accordingly, for each SDR and thus each pressure rating, each size of pipe will have a different specified minimum wall thickness and close tolerances within which this thickness shall be kept. The outside diameter, which is the actual outside diameter, is referred to as the "nominal outside diameter" as opposed to the nominal size, which is the rounded off numerical diameter for pipe sizes up through a nominal size 12-inch pipe. The outside diameter is larger than the nominal size for nominal sizes through 12-inches, which has a nominal outside diameter of 12.750. However, above 12 inches the nominal size and the nominal outside diameter are equal. Since, as indicated, the pressure rating of a specific pipe is determined by the SDR, the following formula for obtaining the pressure rating of sewer pipe at 73.4.degree. F. can be utilized:
P=2S/SDR-1 (2)
where:
In addition to the pressure rating and standards set forth to accomplish this end, specific plastics require different dies in the formation of the pipe. For example, a plastic, such as polyethylene, which has an appreciable amount of memory, requires a die having an elongated heated section, which actually is the main pipe forming section of the die. This section of the die is also referred to as the "land" and its length as the "land length", and is an elongated, substantially straight annular space formed between an elongated outside bushing ring and an elongated inside mandrel. In order to produce a given size pipe meeting the specifications for a given SDR, experience with the use of a given plastic teaches one that a die should have a land length to die opening ratio of approximately a predetermined value, where the die opening is the thickness of the annular space between the outer bushing and the inner mandrel. For example, for a particular polyethylene, it has been found that a land length to die opening ratio of approximately 35:1 must be met for efficient extrusion and the production of pipe meeting standard specifications.
It is also obvious that, in the extrusion of large diameter pipe, the cost as well as the handling of heavy metal dies, particularly those having long land lengths, becomes a serious problem. This problem is aggravated by the fact that a given diameter pipe, having a given SDR value, requires a specific die opening and thus each die can be used only for making a single diameter pipe having a single SDR. As indicated, for large diameter pipes, this becomes a real problem because of the cost of such dies. As a result, such dies, for producing large diameter pipes, are often shipped from one manufacturing plant to another of the same owner, rather than to purchase more than one die for each size pipe having a particular SDR rating. Another cost saving procedure, followed by some pipe manufacturers, to overcome this problem is to have a die built for a single pipe diameter and a plurality of inside bushings which can be substituted for one another each time a different die opening is desired to produce pipe of that particular outside diameter but having different SDR ratings. As previously indicated, the internal mandrel is a heavy metal elongated ring. Removal and replacement of the inner mandrel is not a serious problem with small diameter pipes below about 6 inches, since the mandrel will have an outside diameter less than 6 inches and a length less than about 27 inches. However, for larger extrusion dies, such as those for the production of 18 -inch and 24-inch pipe, a serious problem exists since the mandrel will be nearly 18 or 24 inches in diameter and as long as 98 inches and up.
In practice of the previously mentioned procedure of removing and replacing the entire mandrel, for example, utilizing a set of three mandrels for making 24-inch pipe having SDR ratings of 32.5, 21 and 11, respectively, the die is designed for a mandrel for the thinnest pipe (SDR 32.5) and replacement mandrels of the same length are supplied for the heavier walls required for SDR 21 and SDR 11 pipes. The result of this design is that a 24-inch die designed for extrusion of a plastic requiring a 35:1 land length to die opening ratio (for an SDR 32.5) will result in the modified die for producing SDR 21 pipe having an effective land length to die opening ratio of 22:1 and the modified die for making SDR 11 pipe having an effective land length to die opening ratio of 12:1. Obviously, the large variations from the required land length to die opening ratio of 35:1 can create very serious problems, both in the formation of the pipe by the die, as well as the production of pipe meeting the various standards and specifications.
It is, therefore, an object of the present invention to provide an improved die assembly for the extrusion of thermoplastic pipe. Another object is to provide an improved die assembly for the extrusion of thermoplastic pipe, wherein a single basic die can be converted for the production of a specific diameter pipe having different SDR ratings. Another and further object of the present invention is to provide an improved die assembly for the extrusion of thermoplastic pipe from a plastic exhibiting an appreciable amount of memory. A still further object of the present invention is to provide an improved die assembly for the extrusion of thermoplastic pipe which is adapted to produce a given diameter pipe having a plurality of different SDR ratings, which die closely approximates a predetermined land length to die opening ratio for each of the different SDR ratings. Yet another object of the present invention is to provide an improved die assembly for the extrusion of thermoplastic pipe which is capable of being converted to the production of pipe of a given diameter but having different SDR ratings by replacement of a single relatively small element with a minimum of part removal and replacement. Another further object of the present invention is to provide such an improved die assembly which can be converted for use with plastics having different degrees of shrinkage.